The present invention concerns an improved method for preparing aluminum nitride powders. The present invention also concerns an apparatus suitable for use in conjunction with the improved method.
Aluminum nitride exhibits certain physical properties which make it particularly suitable for use in a variety of applications. Some applications, e.g., packaging components for electronic circuitry, require substantially full theoretical density and high thermal conductivity. High quality aluminum nitride powder, when densified by sintering, hot-pressing or other suitable means, generally satisfies these requirements. A number of factors contribute to powder quality. Powder particle size and surface area primarily affect density of the resultant ceramic article. Powder purity plays a major role in determining purity of the resultant ceramic and thereby the magnitude of certain physical properties such as thermal conductivity.
High quality aluminum nitride powder typically has a low oxygen content (less than about 2%), a low carbon content (less than about 0.2%), and low trace metals content (less than a few hundred parts per million). Lower quality aluminum nitride powders, e.g., those with greater oxygen, carbon or trace metals contents, are generally regarded as unsuitable for use in certain electronics applications such as electronic packaging. Sinterable aluminum nitride powders typically have a particle size of from 1.0 to 0.2 micrometers inclusive. The surface area of the powders, being inversely proportional to the particle size, ranges from about 2 to 10 m.sup.2 /g inclusive.
Production of aluminum nitride powder typically follows one of two known methods. One method, known as direct nitridation, involves nitriding of metallic aluminum nitride powder in a nitrogen or ammonia atmosphere at high temperature and pulverizing the resultant nitride. The second method, known as carbothermal reduction, reacts aluminum oxide, carbon and nitrogen at a high temperature. The present invention focuses upon the latter method.
An examination of the carbothermal reduction reaction thermochemistry shows that it has a highly endothermic nature under all conditions. As such, heat must be supplied in an effective and efficient manner if the reaction is to proceed at an acceptable velocity. Adverse effects of an improper supply of heat include an incomplete reaction of starting materials, coarsening or grain growth of the aluminum oxide starting material or the aluminum nitride product or both, and undesirable side reactions to form unwanted byproducts such as aluminum oxynitride.
Complete conversion of the reactants requires both an effective introduction of reactant gases, e.g., nitrogen, into the reacting mass and an efficient removal of product gases such as carbon monoxide therefrom. If reactant gas introduction and product gas removal are not done properly, the resultant aluminum nitride product can contain high levels of oxygen. Excess oxygen indicates that the reaction has achieved an equilibrium position between starting materials and products which lies short of complete conversion to the desired aluminum nitride.
Kuramoto et al. (U.S. Pat. No. 4,618,592) teach the importance of choosing and maintaining high purity in the reactant solids, e.g., aluminum oxide and carbon. They also teach the importance of preparing an intimate mixture of the reactant solids. Their Example 1 discloses a small (30 to 200 gram) scale reaction in an electric furnace operating at about 1600.degree. C. while feeding nitrogen gas into the furnace at a rate of 3 liters per minute. Following a reaction time of 6 hours, the mixture is removed and oxidized in air to remove unreacted carbon.
Reaction conditions suitable for use in conjunction with a laboratory scale reactor may not provide acceptable results in a larger scale apparatus. A small reacting mass allows for relatively efficient gas and thermal transport which, in turn, lead to preparation of high quality powders even under far less than ideal conditions. As the size of the reaction vessel increases to accommodate larger reacting masses, degradation of gas and thermal transport efficiency usually follows. As the depth of a bed of reactant solids increases, difficulties in providing contact between reactant solids and reactant gases and removal of product gases change from minor irritants to major problems. As the bed of reactant solids increases in size, heating of the bed to drive the endothermic reaction toward completion becomes increasingly non-uniform and varies with the distance of a portion of the bed from the source of heat. In other words, the reaction proceeds from the outside of the reactant bed or charge toward its center in response to an external source of heat. The foregoing gas and thermal transport problems give rise to less than ideal reaction conditions in local volumes within a reacting mass and consequent variability in aluminum nitride conversion and quality.
Design and operation of a reactor or process to provide near ideal reaction conditions in the reacting mass, while necessary, are not sufficient for a successful scale-up of aluminum nitride synthesis to industrial scale. Other factors, including raw materials, labor and utilities, must be managed efficiently in order to manufacture a competitive product.
Although operation of a continuous reactor or process may provide a cost effective use of utilities and labor, it also necessarily implies motion or moving parts. The design and operation of a reactor with hot moving parts is limited by the availability and performance of suitable materials. Addressing this limitation, while necessary, may give rise to other problems such as unacceptable loss of reaction control and product quality.
A number of references describe reactors and processes for preparing aluminum nitride Some suggest the potential for industrial scale reaction of aluminum oxide with carbon and nitrogen. Others address reaction scale without reference to the product quality. Although a few references bring up the need for complete conversion of reactants to a product containing low oxygen, none address industrial scale facilities and processes for preparing high quality aluminum nitride having both low oxygen and fine particle size. Indeed, references which stress scale and product oxygen content necessarily preclude attainment of a fine particle size material.
Kuramoto et al., supra, disclose a process which prepares high quality powder. The process is not, however, suitable for practice on an industrial scale. Static beds of powdered solid reactants are impractical for large scale operations due to problems with product quality and uniformity and uneconomical reaction kinetics. Reaction times of six hours or more are clearly excessive.
Serpek (U.S. Pat. No. 888,044) discloses a method of producing aluminum nitride which consists of heating a mixture of alumina, carbon and a metal capable of forming an alloy with aluminum in a nitrogenous atmosphere to red heat. The resultant product quality is less than desirable because of contamination due to retained metal.
Serpek (U.S. Pat. No. 1,030,929) teaches the use of an electric furnace in which raw material powder mixtures are introduced into a rotary reaction chamber heated by resistance elements. Conversion of the mixtures to aluminum nitride is assisted by a counter flow of gaseous nitrogen. The rotary action of the chamber provides necessary agitation of the powder mixtures. This facilitates both gas and thermal transport. However, it also leads to unacceptable mixing of unreacted, partially reacted and fully reacted materials. If the reactor is operated at feed rates and residence times sufficient to fully convert all of the unreacted materials in such a mixture, the resultant material is still not uniform. The lack of uniformity translates to an unacceptable product.
Serpek (U.S. Pat. No. 1,078,313) teaches incorporation of hydrogen into the nitrogenous reaction atmosphere to induce somewhat faster initial reaction kinetics. However, the best product shown in the examples contains only 8.6 percent nitrogen, an indication of a conversion of approximately 30 percent.
Shoeld (U.S. Pat. No. 1,274,797) teaches a process for producing aluminum nitride which utilizes a vertically situated reaction zone through which briquets of aluminum oxide and carbon and a binder are passed while a nitrogen containing gas is uniformly distributed within. The reacting mass is heated by means of electrodes which cause current to pass through the briquets, heating each directly and uniformly. The configuration and operation of this process places severe demands upon the composition and physical properties of the feed briquets and on the partially and completely reacted briquets as well. In order for the briquets to pass electricity, the composition must be precisely tailored to provide the correct resistance. Unfortunately, the resistance clearly changes in an unpredictable fashion as the material is reacted. This unpredictability leads to inefficient heating of the reacting mass which, in turn, leads to variable reaction kinetics and nonuniform product quality. In addition, a vertical deep bed of briquets places severe constraints on briquet strength. The briquets must have both high unreacted strength and sufficient strength during conversion to avoid disintegration and consequent blinding of the column to flow of gaseous nitrogen. High strength is usually provided by incorporation of large amounts of binder or by the preparation of a denser material. Large amounts of binder compromise the purity of the product or change the course of the reaction whereas denser feed briquets inhibit the necessary gas transport within the briquet resulting in longer reaction times or lower product quality or both.
Perieres et al. (U.S. Pat. No. 2,962,359) teach the importance of maintaining effective control of the atmosphere flow and composition in all portions of the reacting mass including the volume within individual porous briquets. The briquets consist of aluminum oxide and aluminum oxide in admixture with coke. Perieres et al. also teach the existence of volatile solid byproducts which can clog the reactor and otherwise alter the reaction's critical stoichiometry.
Clair (U.S. Pat. No. 3,032,398) discloses a process for continuously producing aluminum nitride. The process comprises forming a particulate feed material composed of aluminum oxide, carbon and a calcium aluminate binder; continuously passing the particulate material downward into an externally heated elongated reaction zone; passing a countercurrent flow of nitrogen through the descending particulate material; and removing and recovering the aluminum nitride below the reaction zone. The exhaust gases are conducted through an expansion zone to condense any calcium contained in the gases. The volatilized calcium compounds, if not removed, would otherwise clog the reactor. Some calcium remains in the product and represents an undesirable impurity. The binder also causes excessive sintering of particulate material thereby preventing recovery of a fine particle size product. Because nitrogen is consumed in the reaction and carbon monoxide is released, the elongated reaction zone with its axial flow of gas necessarily contains a non-uniform reaction atmosphere. In addition, the mechanical nature of the particulate flow within a stationary tube results in a nonuniform distribution of particle velocities leading to an uncertain residence time. Furthermore, countercurrent gas typically flows via channels within a deep or elongated bed. Such flow patterns contributes to production of a nonuniform product.
Paris et al. (U.S. Pat. No. 3,092,455) disclose a process for producing aluminum nitride wherein aluminum oxide grains are contacted with a reactant gas containing a hydrocarbon as a source of carbon. The process may be used in conjunction with a fixed bed reactor, a moving bed reactor, or a fluidized bed reactor. The introduction of a hydrocarbon into a fixed or moving bed of aluminum oxide grains, in either a co-current or a countercurrent flow, results in a nonuniform distribution of carbon, a critical reactant. The resultant product is expected to be similarly nonuniform. The fluidized bed typically provides for rapid and uniform mixing of the solid and gaseous reactants. However, continuous operation of a fluidized bed mandates continuous removal of product. The product so removed contains a finite, but undesirable, amount of unreacted and partially reacted solids.